1. Field of the Invention
The present invention relates generally to wobble plate compressors and more specifically to a wobble plate compressor which is adapted for use in an automotive air conditioner refrigeration system and which features a control arrangement which improves the performance characteristics of the same.
2. Description of the Prior Art
In automotive air conditionining units and the like, the refrigeration unit forming an essential part thereof, includes an arrangement wherein the high temperature/pressure gas which is discharged from the compressor, is cooled and liquified by heat exchange with the ambient atmosphere. Following this the liquid refrigerant or working fluid is fed into an evaporator in the form of a mist after being reduced in pressure by an expansion valve. Heat from the air being "conditioned" is absorbed by the mist which is subsequently converted into its gaseous state. However, in automotive applications, the compressor is subject to frequent change due to the corresponding change in rotational speed of the engine (ICE) by which it is driven. This varies the thermal load applied to the evaporator.
In order to compensate for this rotational speed and thermal load variation the refrigerant valve is commonly used to control the amount of refigerant that is flowing into the evaporator to a level appropriate for the instant set of operating conditions. Viz., as there is a given relationship between the evaporative temperature and the evaporative pressure in the evaporator, by controlling the amount of liquid refrigerant which enters the evaporator the temperature thereof can also be controlled. Accordingly, it is possible to control the temperature of the evaporator with respect to the thermal load on the system.
In order to prevent the water which condenses on the evaporator from freezing it is known to use a thermoswitch which is responsive to the surface temperature of the evaporator and which stops the operation of the compressor in the event that the surface temperature lowers excessively.
However, it is often the case that the air conditioning system is used in spring, winter and autumn to de-humidify the air within the vehicle cabin and prevent fogging of the windows and the like. Under these conditions the thermal load on the system is lower than in summer due to the inherently lower ambient air temperature and the compressor is subject to frequent ON/OFF operation.
Further, in the case of a non-variable type compressor the amount of refrigerant discharged by the same does not change with thermal load and thus consumes the same amount of power even when the thermal load is low.
To obviate this inefficient operation it has been proposed to use a variable displacement wobble plate compressor of the nature disclosed in JP-A-58-158352 (which corresponds to United States Patent No. 4,428,718 issued on Jan. 31, 1984 in the name of skinner). In this arrangement the displacement of the compressor is varied in accordance with the induction pressure. This allows the amount of refrigerant which flows through the system to be controlled with respect to the thermal load on the evaporator while maintaining the induction pressure constant.
With this arrangement the undesired phenomenon wherein, due to over throttling by the expansion valve, the evaporative pressure is excessively lowered, is avoided. This obviates the freezing of condensed water on the evaporator and obviates the need to provide a thermoswitch.
However, this arrangement has suffered from the drawback that as the compressor output is controlled with respect to its induction pressure and in a manner which prevents the freezing of the condensed water on the evaporator surface, it is not possible to vary the temperature of the evaporator. By way of example, the above mentioned wobble plate arrangement is so controlled that the induction pressure assumes a value of about 2.1 Kg/cm.sup.2. Accordingly, as the induction port of the compressor is connected to the evaporator, the temperature of the latter varies with the evaporative pressure whereby the water condensed on the surface of the evaporator is prevented from freezing. During the summer, even when the temperature of the evaporator is set below 0.degree. C., the higher ambient air temperatures prevent the freezing of the condensed water. However, when the ambient air temperature lowers such as during winter, the set induction pressure of 2.1 Kg/cm.sup.2 requires the operation of the compressor to be frequently stopped as the difference between the desired and ambient air temperatures is small.